Antibody-induced modulation of CD26 surface expression.

The ability of different anti-CD26 monoclonal antibodies to modulate the expression of CD26 on human T lymphocytes was investigated. By means of a new non-radioactive method using fluorescein isothiocyanate (FITC)-labelled and unlabelled anti-CD26 monoclonal antibodies and flow cytometry, we measured the internalization and re-expression of CD26 on freshly isolated resting human T lymphocytes. The modulation of CD26 surface expression takes place in primarily CD26+ as well as in CD26- T lymphocytes, indicating the presence of an intracellular CD26 pool. In fact, with two different anti-CD26 monoclonal antibodies (Ta1 and M5) intracellular CD26 was detected out of which newly expressed CD26 might have originated. This intracellular CD26 pool appears to be maintained by continuous translation of CD26 mRNA.